Why can't I see motion blur in the viewport cycles?
Look here -> http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Cycles/Integrator under Motion Blur ... " Viewport rendering currently will not show motion blur." Not yet but maybe one day ;)

From a technical perspective I'd say - viewport is viewport (and has nothing to do with shutter, focal length, aperture etc.) - If you hit F12 you "jump" to your camera in your scene which has all those specs ... that seems quite comprehensible.

That's because the render process and the viewport rendering are two different things (from a programmers perspective) - that's all I can say ... maybe that will help you What are the technical differences between F12 render and viewport preview?